Actuator arrangement for active vibration isolation comprising an inertial reference mass

ABSTRACT

Actuator arrangement with an actuator ( 8 ), a reference mass ( 22 ), a spring ( 24 ) and a first sensor ( 26; 42, 43; 32, 36 ). The actuator ( 8 ) applies a force between a first object ( 2 ) and to a second object (16; 54). The reference mass ( 22 ) is, in use, supported by a third object ( 16; 56 ) by means of the spring ( 24 ). The first sensor ( 26; 42, 43; 32, 36 ) generates a first distance signal that depends on a first distance (z 2 ; z 5 , z 6 ; z 3 , z 4 ) between the reference mass ( 22 ) and the first object ( 2 ) and is applied to a controller ( 6 ) to actuate the actuator ( 8 ).

The present invention relates to an actuator arrangement for activevibration isolation comprising an inertial reference mass.

FIG. 1 shows an active vibration isolation system according to the priorart. The system comprises a payload 2, which, e.g., may be a metroframein a lithography machine. A velocity sensor 4 is attached to the payload2. Instead of a velocity sensor, an acceleration sensor may be used. Thesensor 4 may be a geophone.

The sensor 4 is connected to a controller 6, sometimes referred to as“sky hook” controller. The controller 6 may be any suitable programmed(micro)computer.

However, analog and digital circuits may be used where appropriate.

An actuator 8 is provided between the payload 2 and “earth” 16. Thecontroller 6 is connected to the actuator 8 to provide the actuator 8with suitable control signals. It is observed that the connectionsbetween the sensor 4, the controller 6 and the actuator 8 are shown asphysical lines. However, as known to a person skilled in the art, theseconnections may be wireless connections. This observation also holds forother connections shown in other embodiments of the present invention.

The actuator 8 is shown in a schematic way. The actuator 8 may be aLorenz motor or any other suitable actuator arranged to generate forcesas controlled by controller 6.

FIG. 1 also shows an airmount 10 comprising a piston 12 and a housing 14in which the piston 12 can move up and down. In use, the housing 14 isfilled with air (or any other suitable gas). A valve 20 is provided thatis connected to the housing 14 by means of a channel 21. A controller isconnected to the valve 20 to control its operation. A sensor 18 isprovided to measure the distance z1 between the housing 14 of theairmount 10 and the payload 2. The sensor 18 is connected to acomparator 17 that also receives a reference signal z1 ref. The sensor18 generates an output signal indicative of the distance z1. Thecomparator 17 generates an output signal that is proportional to thedifference between z1 ref and the output of sensor 18 and applies thisto the controller 19. The controller 19 actuates the valve 20 in such away that the distance z1 is controlled at the desired level z1 ref.

The controllers 6 and 19 need not be separate physical units. They maybe implemented as separate programs running on the same computer.

In practice, the payload 2 may be very heavy, e.g., 3000 kilograms ormore. It is not strictly necessary that the airmount 10 is provided asan actively controlled arrangement. It may, alternatively, be a passivevibration isolation arrangement. Instead of an airmount 10, othervibration isolation arrangements, like a spring, may be used.

In practical situations, as will be evident to a person skilled in theart that there will mostly be three or four airmounts 10 to support thepayload 2. Moreover, FIG. 1 shows one actuator arrangement, includingthe sensor 4, the controller 6 and the actuator 8, however, in practicethere may be multiple actuator arrangements. The actuator arrangementsare then arranged to provide vibration isolation in any of six degreesof freedom (x, y, z and rotations about x, y and z), or combinations ofthe different degrees of freedom.

FIG. 2 a shows the transmissibility of a prior art system as shown inFIG. 1. To clarify FIG. 2 a, reference is made to FIG. 2 c which showsthe payload 2 supported by an active vibration isolation AVI standing onearth 16. The active vibration isolation AVI comprises both the airmount10 and the active actuator 8, as well as the sensors and controllersshown in FIG. 1. The payload 2 may move up and down with an amplitude z,whereas earth 16 may move up and down with an amplitude h. Now, thetransmissibility is defined as the ratio z/h, i.e., as the dependence ofz on h as a function of frequency.

The curve A is an example of this dependence. Curve A shows isolationabove 2 Hz. Below 2 Hz, the payload 2 will simply follow any vibrationof earth 16: the isolation system has a 2 Hz eigenfrequency. As isevident to persons skilled in the art, the eigenfrequency is welldampened.

Now, assume that the active vibration isolation AVI is designed to havean eigenfrequency of 0.5 Hz. Then, as can easily be shown, for avibration with an equal frequency above 2 Hz, the ratio z/h may beroughly sixteen times smaller than in the first case. This betterperformance is shown by curve B in FIG. 2 a.

However, this also has a disadvantage as will be shown with reference toFIG. 2 b. FIG. 2 b shows the compliance of the system as summarized inFIG. 2 c. The compliance is defined as the ratio z/F, where F equals aforce directly acting on the payload (for instance, due to a reactionforce of a moving object on the payload 2). As is known to a personskilled in the art, when one designs the active vibration isolation AVIwith a lower eigenfrequency, this results in a poorer compliance. Forinstance, FIG. 2 b, curve D, shows the compliance z/F for an activevibration isolation with an eigenfrequency of 2 Hz. However, if theactive vibration isolation AVI is designed to have an eigenfrequency of0.5 Hz, this curve D shifts to curve C. One can show that forfrequencies below this eigenfrequency of 0.5 Hz the compliance z/F willbe roughly sixteen times higher than in the earlier case where theeigenfrequency was 2 Hz.

So, FIGS. 2 a and 2 b show that there is a coupling between thetransmissibility and the compliance in the prior art system according toFIG. 1. If one wants to improve the transmissibility by lowering thesuspension frequency, this will be at the cost of the compliance andvice versa.

It is observed that P. G. Nelson, “An active vibration isolation systemfor inertial reference and precision measurement”, Rev. Sci. Instrum.62, (9), September 1991, pages 2069-2075, discloses an activelow-frequency vibration isolation system. Nelson describes aseismometer, comprising a payload to be stabilized and to be isolatedfrom vibrations from earth. The payload is suspended from “earth” bymeans of a first spring. Parallel to the first spring, there is anactuator to damp vibrations.

An additional reference mass suspends from the payload by means of asecond spring. A sensor is provided to measure the distance between thepayload and the additional reference mass. The actuator between thepayload and “earth” is controlled by means of the output signal of thissensor. This document does not disclose a reference mass that issupported by earth and that is used as a reference to be followed by thepayload.

The object of the present invention is to provide an active vibrationisolation arrangement allowing to improve the compliance substantiallywhile at the same time allowing an equal or improved transmissibility.

To that end, the present invention provides an actuator arrangementcomprising an actuator, a reference mass, a first suspension with atleast a predetermined spring characteristic in at least one degree offreedom, and a first sensor, the actuator being arranged to apply aforce between a first object and a second object, the reference massbeing arranged to be supported by a third object by means of the firstsuspension, and the first sensor being arranged to generate a firstdistance signal that depends on a first distance between the referencemass and the first object and to apply the first distance signal to acontroller to actuate the actuator.

In this way, the present invention provides an active vibrationisolation system in which the first object (e.g., a payload) follows theadditional reference mass. The additional reference mass is isolatedfrom the second object (e.g. earth) by means of a suspension. As will beexplained in detail hereinafter, this provides the possibility of anindependent transmissibility and compliance (within the frequency rangewhere the active vibration control operates). Whereas in prior artsystems, the transmissibility and compliance are coupled parameters,they are independent in the setup of the invention. In the prior art,when improving one of these two parameters by changing theeigenfrequency of the isolation system, this improvement is at the costof the other parameter, whereas in the invention both, or only one, maybe improved. The additional reference mass and its suspension can beoptimally designed with respect to its intended function, that is tobehave like a mass-spring system or mass-damper-spring system, whoseresponse only depends on its design and the excitation by the secondobject and where all other parasitic disturbances are avoided as much aspossible.

In one embodiment the actuator arrangement comprises a second sensor formeasuring a second distance between the first object and the secondobject, a first filter connected to the first sensor to generate a firstfiltered output signal, and a second filter connected to the secondsensor to generate a second filtered output signal, the first and secondfiltered output signals to be applied to the controller.

The actuator arrangement according to the invention may comprise ahousing that is arranged to protect the reference mass and thesuspension supporting the reference mass.

In a further embodiment, the actuator arrangement comprises a furthersensor and a filter, the further sensor being arranged to measure afurther distance between the reference mass and the second object and togenerate a further output signal for the filter that is arranged toprovide a filtered output signal, the filtered output signal to beapplied to the controller to allow for compensation of transfer ofvibrations of the second object to the first object.

The suspension may be implemented in any suitable way as known to aperson skilled in the art. One example is that the suspension isimplemented by means of a second actuator, a still further sensor formeasuring a still further distance between the additional reference massand the second object, a further controller arranged to receive a stillfurther output signal from the still further sensor and to actuate thesecond actuator. In this setup, the suspension can be activelycontrolled to have a spring constant as desired depending on designchoices, e.g., periods of time in which the spring needs to be active.

The invention also relates to an active vibration isolation arrangementcomprising at least one actuator arrangement as defined above, andcomprising the controller to actuate at least the actuator.

Below, the invention will be illustrated in detail with reference tosome drawings.

These drawings are only intended to clarify the present invention andshow some embodiments only. They are not intended to limit the inventionin any way. The present invention is only limited by the annexed claimsand its technical equivalences.

FIG. 1 shows an active vibration isolation system according to the priorart;

FIG. 2 a show curves related to transmissibility of the system accordingto FIG. 1;

FIG. 2 b shows curves related to compliance of the system according toFIG. 1;

FIG. 2 c is a very schematic summary of the system according to FIG. 1used to explain FIGS. 2 a and 2 b;

FIG. 3 is an active vibration isolation system in accordance with oneembodiment of the present invention;

FIG. 4 a shows curves relating to transmissibility of the system shownin FIG. 3 and FIG. 1;

FIG. 4 b shows curves related to compliance of the system according toFIG. 3 and FIG. 1;

FIGS. 5 a and 5 b are similar to FIGS. 4 a and 4 b, however related to aspecific arrangement of the system according to FIG. 1;

FIGS. 6 through 12 show different embodiments of the active vibrationisolation system according to the invention.

It is observed that throughout the figures the same reference numbersrefer to the same elements or components.

FIG. 3 shows a first embodiment of the present invention.

In the arrangement according to FIG. 3, the sensors 4, 18 as present inthe arrangement of FIG. 1 have been removed.

An additional reference mass 22 is provided supported on earth 16 by asuspension 24. The suspension 24 may be a spring with a spring constant.However, alternatively, it may be a combination of a spring and a damper(e.g. a sky hook damper) and have a predetermined spring and dampingcharacteristic. Below, for the sake of simplicity, it will mostly bereferred to as spring 24.

In reality, there may be more such suspensions in different degrees offreedom. Then, the mass 22 may be used as a reference mass in multipledegrees of freedom.

A sensor 26 is provided to measure the distance z2 between theadditional reference mass 22 and the payload 2. The sensor 26 sends anoutput signal to a comparator 28. The comparator 28 also receives areference signal zref and subtracts the output signal received from thesensor 26 from zref. An output signal based on this comparison isapplied by the comparator 28 to the controller 6. The controller 6 isconnected to the actuator 8 and may also be connected to the valve 20.

As will be evident to persons skilled in the art, the additionalreference mass 22 does not need to be supported by the spring 24 onearth 16 itself. The additional reference mass 22 may, alternatively, besupported by means of spring 24 on, e.g., a baseframe standing on earth16.

The general idea of the setup of FIG. 3 is that the additional referencemass 22 together with its support can be optimally designed with respectto its intended function, that is to behave like a mass-spring system ormass-damper-spring system, whose response only depends on its design andthe excitation by the second object 16 and where all other, parasiticdisturbances such as friction or crosstalk are avoided as much aspossible.

By designing controller 6 such that the payload 2 follows the positionof additional reference mass 22, the combination of transmissibility andcompliance can be improved dramatically as will be shown with referenceto FIGS. 4 a, 4 b, 5 a and 5 b.

FIG. 4 a shows curves A and B′ which are comparable with the curves Aand B of FIG. 2 a. Curve A shows the transmissibility of the system ofFIG. 1 for an eigenfrequency of 2 Hz. In this case the airmount 10 issaid to be a 2 Hz airmount. Curve B′ shows the transmissibility for anactuator arrangement of FIG. 3 with a reference mass eigenfrequency of0.5 Hz and the airmount 10 also being a 2 Hz airmount. At the lowerright hand side of FIG. 4 a, curve B′ increases but this is determinedby the chosen bandwidth of the control loop.

FIG. 4 b shows compliance curves. Curve D, again, shows the compliancefor the arrangement of FIG. 1 having an eigenfrequency of 2 Hz. Curve Eshows the compliance for the actuator arrangement of FIG. 3. Curve Eshows that the compliance has dramatically improved, i.e., it may beroughly 125 times or more better than curve D up to a frequency thatwill be determined by the control loop.

In figure 5 a, curve B shows the transmissibility of the system as shownin FIG. 1 for an eigenfrequency of 0.5 Hz, the airmount being a 0.5 Hzairmount. In figure 5 a, curve B′ is identical to curve B′ already shownin FIG. 4 a, i.e., it relates to the transmissibility of the actuatorarrangement shown in FIG. 3 with a reference mass eigenfrequency of 0.5Hz and a 2 Hz airmount 10. Thus, FIG. 5 a shows that, due to the choiceof airmount in combination with the limited bandwidth, thetransmissibility of the arrangement according to FIG. 3 may be worsethan in the known arrangement of FIG. 1 for higher frequencies.

FIG. 5 b shows the compliance of the known arrangement of FIG. 1 for aneigenfrequency of 0.5 Hz via curve C. FIG. 5 b also shows curve Ealready presented in FIG. 4 b and which relate to the arrangement ofFIG. 3. Now, one can see that for a prior art system with a lowereigenfrequency of 0.5 Hz, the comparison with the system of FIG. 3 iseven more dramatic than for a prior art system with an eigenfrequency of2 Hz. I.e., FIG. 5 b shows that the improvement may be more than 2000times.

Thus, FIGS. 4 a, 4 b, 5 a and 5 b show that with the setup of theinvention, both the transmissibility and the compliance may be improved.The transmissibility and compliance are no longer coupled as is the casewith the prior art. One can either choose to improve both or only one ofthem without deteriorating the other one. This improvement is possiblebecause the payload 2 now has to follow an independent reference masslike the additional reference mass 22.

It is observed, that in the arrangement according to FIG. 3, thecontroller 6 does not need to control valve 20. The airmount 10 can besubstituted by a passive isolation system like a (large) spring. Afurther alternative may be, that there is no airmount 10 or equivalentspring at all. The actuator arrangement comprising the actuator 8, theadditional reference mass 22, the spring 24 and sensor 26 may beprovided as a separate unit to be applied in any active vibrationisolation system. When applied in a system as shown in FIG. 3 (or otherembodiments still to follow), the additional reference mass 22 may bedesigned with a low eigenfrequency of, e.g., 0.5 Hz. If so, then, theairmount 10 supporting the payload 2 may be designed with aneigenfrequency that is higher, e.g., 2 Hz and may even be passive, whileup to a certain frequency, the payload behaves—with respect to thetransmissibility—as if it were a system with a 0.5 Hz airmount. Such anairmount 10 is cheaper than an airmount 10 with an eigenfrequency of 0.5Hz.

When applied in a system as shown in FIG. 3, with the reference masshaving a 0.5 Hz eigenfrequency, the airmount 10 may also be designedwith a 0.5 Hz eigenfrequency. In this situation, the transmissibilitydoes not change by switching the actuator arrangement on or off, whilethe compliance improves dramatically.

It is observed that the spring 24 may itself be implemented as an activevibration isolation arrangement. For instance, the mass 22 together withthe spring 24 may be designed as the system shown in FIG. 1. Then, theadditional reference mass 22 equals the payload 2, whereas the spring 24is arranged to include sensors 4, 18, valve 20, channel 21, airmount 10(or another spring, possibly passive), actuator 8 and controller 6. Ofcourse, these elements need to be scaled to the desired level then.

FIG. 6 shows a further embodiment of the present invention. In FIG. 6,the spring 24 is implemented by means of an actuator 30, a sensor 32 anda controller 34. The sensor 32 is arranged to measure the distance z3between the mass 22 and the earth 16. The sensor 32 outputs an outputsignal to the controller 34. The controller 34 is arranged to actuatethe actuator 30. The actuator 30 may be any suitable actuator, e.g., aLorenz actuator.

The setup of the actuator 30, the sensor 32 and the controller 34 isarranged such that it behaves as a suspension with a predeterminedspring constant, or with spring and damping constant. E.g., togetherwith the additional reference mass 22, the “mass-spring” system of FIG.6 (comprising the mass 22 and the spring 24) may be designed to have aneigenfrequency of 0.5 Hz. This can easily be done with an active spring24 as shown. Note that such a low eigenfrequency of 0.5 Hz would be verydifficult with a physical spring, taking into account that all parasiticeffects must be prevented or minimized. E.g. a mechanical spring wouldeasily introduce parasitic forces caused by the internal resonances inthe spring itself.

Moreover, the output signal of the sensor 32 may be compared with areference height such that the controller 34 controls the actuator 30 tokeep the mass 22 at a desired offset height for z3. Such a referenceheight provides the arrangement with the option to control the distancez3 in dependence on client requirements. This distance z3 may have adesired value that depends on the desired offset distance between thepayload 2 and the second object 16.

Moreover, the arrangement of FIG. 6 provides the option of changing theeigenfrequency of the additional reference mass 22 and the spring 24during measurements. For instance, it may be that one wishes to have aneigenfrequency of, say, 10 Hz during a certain period of time and aneigenfrequency of 0.5 Hz during a later period of time.

Moreover, the setup of FIG. 6 can be designed such that it has (almost)no hysteresis at all.

It is observed, that spring 24 may also be realized, using a spring witha negative spring constant k as part of the spring 24. This is known toa person skilled in the art and need no further elaboration here.Reference may be made e.g. to website www.minusk.com.

FIG. 7 shows a further embodiment of the present invention.

The arrangement of FIG. 7 comprises, apart from the components/elementsalready described with reference to earlier figures, a sensor 36 formeasuring the distance z4 between the payload 2 and earth 16. The sensor36 is connected to a low pass filter 38. The low pass filter 38 isconnected to comparator 28.

The sensor 26 that measures the distance z2 between the additionalreference mass 22 and the payload 2 is not, as in earlier figures,directly connected to comparator 28 but to a high pass filter 40. Thehigh pass filter 40 is, in tum, connected to the comparator 28.

Because of the low pass filter 38, the output signal of the sensor 36 isdominant for low frequencies, as determined by the filter design.Moreover, the output signal of sensor 26 will mainly influence thefeedback as shown for frequencies above the cut-off frequency of thehigh pass filter 40. Preferably, the cut-off frequencies of the filters38, 40 are related to the eigenfrequency of the additional referencemass 22 with spring 24. Thus, in the arrangement shown in FIG. 7, thepayload 2 will mainly follow movements of earth 16 in the low frequencyrange, whereas the payload 2 will mainly follow movements of theadditional reference mass 22 in the higher frequency range. This setupprovides a solution for the situation where the reference mass is poorlydamped.

Note that the spring 24 can be designed in any way as explained earlier.Also note that the cut-off frequencies of filters 38,40 can bedifferent. Also note that the filters 38,40 can be extended with(multiple) general second-order filters to compensate for specificdynamic effects. Filter 40 may, e.g., be designed such that theinfluence of sensor 26 is minimized in a predetermined frequency range,i.e., filter 40 may be a notch filter. Then, filter 38 may be designedas a bandpass filter such that sensor 36 has a large influence in thatsame frequency range.

FIG. 8 shows a further embodiment of the present invention. in thearrangement according to FIG. 8, a housing 44 is provided around theadditional reference mass 22 and the spring 24. The housing 44 isintended to protect the additional reference mass 22 and the spring 24from external disturbances that may cause the additional reference mass22 to vibrate. An example of such an external disturbance is an acousticsignal.

In the arrangement of FIG. 8, a sensor 42 is provided to measure thedistance z5 between the additional reference mass 22 and an uppersurface of the housing 44. A further sensor 43 is provided to measurethe distance z6 between the upper surface of the housing 44 and thepayload 2. The sensor 42 sends an output signal to the comparator 28.The sensor 43 sends an output signal to the comparator 28 too. As onecan easily see, the sum of distances z4 and z5 has a linear relationshipwith the distance z2 of the earlier embodiments. Thus, the arrangementof FIG. 8, as regards the feedback system, is equivalent to thearrangements of FIGS. 3, 6 and 7.

The housing 44 encloses a space 46. In one embodiment, the space 46 maybe vacuum, i.e. have a pressure below 10⁵ Pa, to further reduce anyinfluence of acoustic signals.

FIG. 8 also shows an alternative embodiment of airmount 10. Thealternative airmount 10′ does not comprise a piston 12 that can beshifted freely in the housing 14 of the airmount 10′, but comprises apiston 12′ that is fixed to the housing 14 by means of a bellowsconstruction, or a membrame construction.

As in the earlier embodiments, the airmount 10′ need not be an activelycontrolled airmount. Alternatively, it may be a passive airmount (or anyother active or passive spring arrangement) or a gravity compensator.Apart from the better possibility to protect the additional referencemass 22 and the spring 24 from external disturbances, the arrangement ofFIG. 8 provides more options as to measuring the distances z5 and z6.

FIG. 9 shows a further embodiment of the present invention.

The arrangement of FIG. 9 is similar to the arrangement of FIG. 8. Thedifference is that sensors 42, 43 are substituted by sensors 36 and 32.The sensor 36 measures the distance z4 between the payload 2 and theearth 16 (as in FIG. 7). The output signal of the sensor 36 is directlyapplied to the comparator 28. The sensor 32 measures the distance z3between the additional reference mass 22 and earth 16 (as in FIG. 6) andsends an output signal corresponding to this distance z3 to thecomparator 28. The comparator 28 adds the output signal of the sensor 32to the reference signal zref and subtracts the output signal of thesensor 36. In this way, the comparator 28 applies an output signal tothe controller 6 that is an indication of z4−z3, that is proportional todistance z2 of the arrangement in accordance with FIGS. 3, 6 and 7.

It is observed that sensors 43 and 36 are technically equivalent: sincethe housing 44 is a fixed stiff body, their output signals differ onlyby a predetermined constant. Sensors 42 and 32 differ also by aconstant, and moreover by a minus-sign.

FIG. 10 shows a further embodiment of the present invention.

The arrangement of FIG. 10 comprises the sensors 32, 42, and 43,respectively, for measuring the distances z3, z5, and z6, respectively,as explained with reference to FIGS. 6, 8. The output signal of thesensor 32 is sent to a multiplier 50 that multiplies the output signalof sensor 32 by a factor−k1. The output signal of the multiplier 50 isapplied to the comparator 28. Thus, the comparator 28 sends an outputsignal to controller 6 that is proportional to z5+z6−k1.z3. In thisembodiment, the signal−k1.z3 is used as a feed forward signal tocompensate for vibrations of earth 16 that may reach the payload 2 viaall kinds of mechanical structures between the payload 2 and earth 16(e.g., cables, cooling water, the airmount, etc.). This compensationsignal can also be used in other embodiments.

Note that, again, z3 and z5 only differ by a fixed constant and aminus-sign. Thus, the arrangement of FIG. 10 can be simplified by usingeither sensor 32 or 42 alone and filtering its output signal in a waysuch that the same effect is obtained.

Moreover, it is observed that multiplier 50 may in general be a filterthat does more than multiplying by −k1. It may e.g. be a low-passfilter.

FIG. 11 shows a further embodiment of the invention.

The embodiment of FIG. 11 resembles the one of FIG. 3. The differencesare as follows.

First of all, the reference mass 22 is not directly supported by earth16. Instead, reference mass 22 is supported by a first subframe 56 viaspring 24. The first subframe 56 is supported by earth 16 via a spring57.

Secondly, the actuator 8 is not directly supported by earth 16. Instead,actuator 8 is supported by a second subframe 54. The second subframe 54is supported by earth 16 via a spring 55.

Thirdly, the airmount 10 is not directly supported by earth 16. Instead,airmount 10 is supported by a third subframe 52. The third subframe 52is supported by earth 16 via a spring 53.

The first, second and third subframes 52, 54, 56 are not connected toone another.

As to the actuator 8, the setup of FIG. 11 has the following advantages.The sensor 26 measures the displacement of the payload 2 relative to thereference mass 22. Based on the output signal of the sensor 26, thecontroller 6 generates a control signal for actuator 8 such that theactuator 8 produces a controlled force to payload 2, as explained above.By doing so the position of the payload 2 relative to the reference mass22 is controlled. Assuming in the setup of FIG. 3 that earth 16 has afinite mass, then earth 16 may be displaced by the force produced by theactuator 8. This latter displacement may, in the setup of FIG. 3, causethe reference mass 22 to move by a force transfer via spring 24.Consequences may be:

-   -   due to the undesired movement of reference mass 22, the        performance of the control system deteriorates: the object is to        have a reference mass 22 that is as stable as possible as to its        position;    -   due to transfer of the force produced by the actuator 8, the        control loop may become unstable. By the displacement of        reference mass 22, the value of Z2 as measured by sensor 26        changes resulting in a further change of the force produced by        actuator 8. To prevent instability of the control loop to occur,        in the setup of FIG. 3, the bandwidth of the control loop has to        be reduced, however, which results in poorer performance.

By adding subframe 54 supporting the actuator 8 and itself beingsupported by earth via spring 55, any force generated by actuator 8 isnot directly transferred to earth 16 but filtered. This results in lessdisplacement of earth 16 and thus less displacement of reference mass 22than in the setup of FIG. 3. Of course, the amount of filtering and theoverall improvement depend on the design choices made as to the mass offirst subframe 54 and the spring constant (and damping) of spring 55.

The improvement achieved by adding subframe 54 and spring 55 can befurther improved by adding subframe 56 that supports reference mass 22via spring 24 and is supported itself by earth 16 via spring 57. Then, afurther filtering effect of the force transfer between actuator 8 andreference mass 22 occurs. It is not strictly necessary that bothsubframes 54 and 56 are additionally applied. An improvement over thesetup of FIG. 3 may also already be achieved by using subframe 56without using subframe 54.

Note that, in a practical situation, there may be four such actuators 8and three sensors 26 with three reference masses 22. These three sensors26 and three reference masses 22 may be implemented as three sensorunits, each sensor unit comprising one sensor 26 and one reference mass22. These sensor units and actuators 8 may be arranged remote from oneanother.

Finally, in addition to or as a separate measure to the setup of FIG. 3,the airmount 10 may be supported on the subframe 52, preventing directforce transfer from earth 16 to airmount 10 due to any displacement ofearth 16.

FIG. 12 shows a further embodiment of the invention.

In the arrangement of FIG. 12, the setup of FIG. 3 is combined with skyhook arrangement as already explained with reference to FIG. 1. Thesensor 4 is shown to produce an output signal for a controller 62. Forcompleteness′ sake, the output signal is shown to be applied to acomparator 60, which compares this output signal with a reference signalZref,a. The comparator 60 produces an output signal for controller 62.The controller 62 produces a control signal in dependence on the signalreceived from the comparator 60. That control signal is added to thecontrol signal produced by controller 6 in a summing device 58. Thusboth control signals of both controllers 6, 62 control the force to begenerated by actuator 8. As will be evident to persons skilled in theart, there need not be two separate controllers 6, 62. The requiredfunctionality can be provided by one single controller, programmed in asuitable way.

The prior art arrangement of FIG. 1 may be a low-frequency suspensionsystem with a suspension frequency in the range from 0.5-5 Hz. In thearrangement according to FIG. 12, however, due to the control loop viasensor 26 and controller 6, the suspension frequency may be much higher,e.g., in the range between 20-80 Hz. If so, then, in the arrangement ofFIG. 12, the sensor 4 has only to be suitable for functioning in asystem with a suspension frequency of typically 20-80 Hz. In practicethis is an easier task: using sensor 4 in the range from 0.5-5 Hz needsadditional measures, like using a stretch filter as known to personsskilled in the art. In the arrangement of FIG. 12, such a filter can beomitted.

As will be evident to persons skilled in the art, the invention is notrestricted to the embodiments described above. Several alternatives arepossible.

For instance, the arrangements may, in reality, be upside down relativeto the arrangements shown. Moreover, the controllers 6, 34 are shown asseparate units. In reality they may be implemented as separate programson the same computer. Sensors measuring the same distance are indicatedwith the same reference numerals. However, they may be different. Thefilters 38, 40 may be part of controller 6. They may be implemented asanalogue filters, digital filters or part of a program running on acomputer. The comparator 28 need not be a separate unit but may beintegrated in controller 6, either as unit or as part of a computerprogram. The comparator 60 need not be a separate unit but may beintegrated in controller 62, either as a unit or as part of a computerprogram. The multiplier 50 may similarly be an integral part ofcontroller 6. The airmount 10, 10′ may be substituted by a 5 gravitycompensator. The sensors may be capacitive sensors or interferometers.Amplifiers may be arranged between the controllers and the actuators, asrequired.

Moreover, different parts of the different embodiments may be combined.For instance, in the embodiment of FIG. 9, the housing 44 may beomitted. Also in FIG. 10, the housing 44 may be omitted. Then, sensors42, 43 will be substituted by sensor 26.

Moreover, there may be more than one airmount to support the payload 2.The additional reference mass 12 may be suspended in more than onedegree of freedom and may then be a good reference for measuring theposition of the payload 22 in more than one degree of freedom. Then,there may be more sensors provided to measure distances between thepayload 2 and the reference mass 22 to obtain information aboutdistances and rotations in more degrees of freedom. The outputs of thesesensors are applied to a multi-input-multi-output processor thatcontrols several actuators (via suitable amplifiers) actuating thepayload 2 in the desired degrees of freedom. Instead of one additionalreference mass 22 with suspension 24, there may be provided a plurality.All these alternatives/additions are included in the scope of theannexed claims.

1. Actuator arrangement comprising an actuator (8), a reference mass(22), a first suspension (24) with at least a predetermined springcharacteristic in at least one degree of freedom, and a first sensor(26; 42, 43; 32, 36), the actuator (8) being arranged to apply a forcebetween a first object (2) and a second object (16; 54), the referencemass (22) being arranged to be supported by a third object (16; 56) bymeans of said first suspension (24), and said first sensor (26; 42, 43;32, 36) being arranged to generate a first distance signal that dependson a first distance (z2; z5,z6; z3,z4) between said reference mass (22)and said first object (2) and to provide said first distance signal to acontroller (6) to actuate said actuator (8).
 2. Actuator arrangementaccording to claim 1, wherein said actuator (8) is a Lorenz actuator. 3.Actuator arrangement according to claim 1, wherein said actuatorarrangement comprises a second sensor (36) for measuring a seconddistance (z4) between said first object (2) and said second object (16),a first filter (40) connected to said first sensor (26) to generate afirst filtered output signal, and a second filter connected to saidsecond sensor (36) to generate a second filtered output signal, thefirst and second filtered output signals to be applied to saidcontroller (6).
 4. Actuator arrangement according to claim 1, whereinsaid actuator arrangement comprises a housing (44) arranged to protectsaid reference mass (22) and said first suspension (24) and having asurface to be located between said first object (2) and said referencemass (22), said first sensor comprising a first subsensor (42) and asecond subsensor (43), said first subsensor (42) being arranged tomeasure a third distance (z5) between said reference mass (22) and saidsurface of said housing (44), and said second subsensor (43) beingarranged to measure a fourth distance (z6) between said surface of saidhousing (44) and said first object (2).
 5. Actuator arrangementaccording to claim 1, wherein said first sensor (26) comprises a firstsubsensor (36) and a second subsensor (32), the first subsensor (36)being arranged to measure a third distance (z4) between said firstobject (2) and said third object (16; 56) and to generate a third outputsignal, the second subsensor (32) being arranged to measure a fourthdistance (z3) between said reference mass (2) and said third object (16;56) and to generate a fourth output signal, the fourth output signal tobe subtracted from said third output signal before delivery to saidcontroller (6).
 6. Actuator arrangement according to claim 5, comprisinga housing (44) to protect said reference mass (22) and said firstsuspension (24).
 7. Actuator arrangement according to claim 1,comprising a further sensor (32) and a filter (50), the further sensor(32) being arranged to measure a further distance (z3) between saidreference mass (22) and said third object (16; 56) and to generate afurther output signal for said filter (50) that is arranged to provide afiltered output signal, the filtered output signal to be sent to saidcontroller (6) to allow for compensation of transfer of vibrations ofsaid second object (16) to said first object (2).
 8. Actuatorarrangement according to claim 1, wherein said first suspension (24) isimplemented by means of a second actuator (30), a still further sensor(32) for measuring a still further distance (z3) between said referencemass (22) and said third object (16; 56), and a further controller (34)arranged to receive a still further output signal from said stillfurther sensor (32) and to actuate said second actuator (30). 9.Actuator arrangement according to claim 1, wherein said second objectand said third object are one and the same.
 10. Actuator arrangementaccording to claim 1, wherein said third object (56) is supported bysaid second object (16) by a second suspension (57), said actuator issupported by a fourth object (54), and the fourth object is supported bysaid second object by a third suspension (55).
 11. Actuator arrangementaccording to claim 1, wherein said actuator arrangement comprises a skyhook control arrangement (4, 62) for controlling displacement of saidfirst object (2).
 12. Active vibration isolation arrangement comprisingat least one actuator arrangement according to claim 1, and comprisingsaid controller (6) to control at least said actuator (8).
 13. Activevibration isolation arrangement according to claim 9, comprising atleast one airmount (10; 10′) to support said first object (2).